On the 2008 Impreza there is no mechanism for adjusting rear camber.
Whiteline has solved this problem with the KCA326. These unique bushings replace the stock units in the upper control arm. How they achieve adjustment is through an off-center bored crush tube. By doing this they essentially have created a cam inside the bushing.
This "cam" is good for about +/- .75* of adjustment when compared to the stock set up